Conventionally, FR-4 type laminates obtained by curing epoxy resins with dicyandiamide have been widely used for laminates for printed wiring boards. However, it is difficult to cope with a demand for high heat resistance by means of this technology. Cyanate ester resins are known as resins having excellent heat resistance for printed wiring boards. In recent years, prepregs comprising resin compositions containing bisphenol A type cyanate ester resins and other thermosetting resins or thermoplastic resins are widely used for laminates for semiconductor plastic packages.
The above bisphenol A type cyanate ester resins have excellent properties in terms of electrical characteristics, mechanical properties, chemical resistance and adhesive properties. However, the bisphenol A type cyanate ester resins are insufficient under severe conditions in terms of moisture resistance or heat resistance after moisture absorption in some cases. Therefore, studies of cyanate ester resins having other structures have been made for the purpose of further improvements in properties.
Phenol novolak type cyanate ester resins are mostly used as the cyanate ester resins having other structures (JP-A-11-124433). However, the phenol novolak type cyanate ester resins have small cyanate group equivalents and many unreacted cyanate groups are apt to remain at the time of curing because of their stiff skeleton structure. Therefore, the phenol novolak type cyanate ester resins are insufficient in terms of properties such as adhesion to metal foil, moisture resistance and heat resistance after moisture absorption.
Further, the use of naphthol aralkyl type cyanate ester resins has been also studied (JP-A-2007-45984). Resin compositions containing such cyanate ester resins can retain heat resistance owing to their stiff resin skeleton structure and curability is increased by decreasing reaction inhibition factors, so that the resin compositions have excellent properties such as moisture resistance and heat resistance after moisture absorption.
Furthermore, high integration, high functionalization and high-density packaging of semiconductors, which are widely used for electronic equipment, communication apparatuses and personal computers, have been increasingly accelerated. Semiconductor plastic packages unfold from QFP to area mounting type semiconductor plastic packages such as BGA and CSP. Further, high-functional semiconductor plastic packages such as MCP and SIP have appeared. As above, the form of semiconductor plastic packages is becoming various. Therefore, it is more strongly required than ever that laminates for semiconductor plastic packages have high reliability.
The coefficient of thermal expansion of semiconductor elements is 3 to 6 ppm/° C. It is smaller than the coefficient of thermal expansion of general printed wiring boards for semiconductor plastic packages. For this reason, when a semiconductor plastic package undergoes thermal shock, warping occurs in the semiconductor plastic package due to the difference in coefficient of thermal expansion between a semiconductor element and a printed wiring board for the semiconductor plastic package, which sometimes causes defective connection between the semiconductor element and the printed wiring board for the semiconductor plastic package or between the semiconductor plastic package and the printed wiring board mounted therein. It is necessary to develop laminates having a small thermal expansion coefficient in plane direction for decreasing the warping for the purpose of securing connection reliability.
As a method for decreasing the coefficient of thermal expansion of laminates, there is a method in which an inorganic filler is incorporated. When the amount of the inorganic filler to be incorporated is large, an obtained resin composition is fragile. This causes a deterioration in the quality of drilling processing performed for making through holes necessary for layer-to-layer connection of printed wiring boards. Further, a drill bit used for the drilling processing wears out in a very short period of time, which results in an extreme decrease in productivity of the drilling processing. As a means for lowering thermal expansion in plane direction, it is known that organic fillers having rubber elasticity are incorporated into varnishes containing epoxy resins (Japanese Patent No. 3173332, JP-A-8-48001, JP-A-2000-158589, JP-A-2003-246849 and JP-A-2006-143973). When the above varnishes are used, it is required to use bromic flame-retardants for rendering laminates flame-resistant.
Conventionally, bromic flame-retardants are used for imparting flame retardancy to laminates. However, resin compositions containing no halogen compound are desired in accordance with a recent growing interest in environmental issues. Further, phosphorus compounds have been studied instead of halogen flame-retardants. However, there is a danger that the phosphorus compounds generate toxic compounds such as phosphine at the time of combustion. Therefore, it is desired to develop laminates having flame retardancy and a low thermal expansion coefficient without halogen compounds and phosphorus compounds.